The cut-off valve is a valve with a line sealing opening and closing of the complex working conditions in high and low long-term change, sealing the high and low temperature thermal cycling condition; at the same time, the use of the valve body of the non-metallic sealing materials shall be in the refrigerant, lubricant, corresponding metal materials and mixture should be good compatibility; otherwise, the valve can not guarantee the reliability and performance of the. There are two ways to install the cut-off valve. One is the medium flowing from the top of the spool. The advantage is that the packing is not stressed when closing. It can replace the packing when the valve is pressed, and has long service life. The other is the medium entering from the bottom of the spool. Its disadvantage is that the driving torque is very large, which is 1 times that of the upper part. The axial force of the stem is large, and the stem is easy to bend. Generally suitable for small caliber manual cut-off valve, with the shut-off valve closed when the force of the valve core is not more than 350kg. The electric cut-off valve is generally used in the way of medium from above.